1. Field of the Invention
This invention relates to a storage control system connected to a higher-level device and to a method of control of this system.
2. Description of the Related Art
In for example a storage system for basic operations handling large amounts of data, a host computer (hereafter simply “host”) manages data using a separately configured storage control subsystem. This storage control subsystem may for example also be called a storage control system, and is a RAID (Redundant Array of Independent Inexpensive Disks) configured with numerous disk storage devices arranged in an array.
For example, in Japanese Patent Laid-open No. 11-224164, a disk subsystem is disclosed having a reduced logical volume and normal logical volume, which manages cache memory using an LRU method (LRU methods are discussed in the paragraph following the next), and which, by changing the position for addition of LRU data columns for data in the reduced logical volume and the position of data in the normal logical volume, is able to cause data of the reduced logical volume to remain a long time in the cache memory.
However, there exist storage control systems in which, among a plurality of logical storage devices (hereafter abbreviated to “logical devices” or “LDEVs”) prepared from one or a plurality of disk storage devices, one storage unit, comprising one or more LDEVs, is provided to the higher-level device as an LU (logical unit). In such a storage control system, for example, an LU pair may be formed in which one LU is a primary LU and another LU is a secondary LU, and un-updated data in the primary LU copied to the secondary LU (that is, a so-called snapshot is created).
For example, the storage capacities of both LUs comprised in the LU pair are the same. Hence if the storage capacity of the primary LU is large, the storage capacity of the secondary LU must also be large. In general in a storage control system, the same storage capacity as the LU size provided to the higher-level device is allocated from disk storage devices. However, in general the storage capacity actually used by a higher-level device is only a small portion of the storage capacity actually allocated, and in some cases most of the secured storage capacity is idle. When formatting an LU, data of size equal to the LU storage capacity is output from the host terminal, and the data is written to the entire LU.